For each pickup cavity- One strip of foil affixed to floor- Rest of floor and walls painted with graphite paint (overlapping onto foil for electrical continuity)- A wire soldered to the foil strip and the volume pot

Now she’s dead silent - no buzz when I take my hands off the metal parts.

I was quite surprised since only the floors and walls of the pickup cavities are shielded - they are 'open sky' so to speak...

I thought you’d find that interesting!

So you were right - on rear routed instruments, graphite painting the pickup cavities really helps.

I suppose you can think of them as a Faraday cage with a hole in. The effectiveness of a Faraday cage (note the word cage rather than solid enclosure) comes down to the largest sized hole it it, which corresponds roughly to the shortest wavelength it will stop entering. So a hole that size will be good for most radio waves down to the start of microwaves.